Foams of polyester resins have been formed by first producing an emulsion of water and polyester resin and then adding blowing agents to the emulsion in the form of bicarbonates. The emulsion formation is considered necessary to enable rapid dispersion of the blowing agent through the resin before the blowing reaction or resin curing have advanced too far.
The foams which are produced by the latter procedure have an open cell structure to enable them to be dried for removal of excess water remaining after foaming. The water also has an adverse affect on the foam-forming reaction, lowering the peak temperature, the foam strength and impact resistance.
For these reasons, polyester foams have not achieved widespread commercial use, and the principal use of polyester resins in structural work has been in glass fiber laminates made up of successive cured layers which are built up to provide stiffness and bending resistance.